what units are inserted to make door and window openings in concrete wall forms?\nbuck\nspurs\nspacers\nbacke…

Brief Explanations:

In construction, bucks are inserted into concrete - wall forms to create door and window openings. Spurs are used for other purposes like securing formwork, spacers maintain rebar position, and backers are related to finishes or support in other contexts.

Answer:

A. buck